locomotory posture, just as important may have been the need to be able to breathe while walking. Like a lizard, Proterosuchus may still have had a back-and-forth sway to its body as it walked, and this sort of locomotion causes compression on the lung area due to what is known as Carrier’s Constraint—the concept that quadrupeds with splayed out legs, such as most (but not all!) lizards, cannot breathe while they run, because their sinuous, side-to-side swaying impinges on the lungs and rib cage, inhibiting inspiration.
For this reason, most lizards and salamanders cannot breathe while walking, and Proterosuchus may have had something of this effect, although not as pronounced as in modern-day salamanders or lizards. A solution is to put the legs beneath, but this is only a partial solution. To truly be free of the constraint that breathing puts on posture, extensive modifications to the respiratory system and the locomotory system had to be made. The lineage that led to dinosaurs and birds found an effective and novel adaptation to overcome this breathing problem: bipedalism. By removing the quadruped stance, they were freed of the constraints of motion and lung function. The ancestors of the mammals also made new innovations, including a secondary palate (which allows simultaneous eating and breathing) and a complete upright (but still quadruped) stance. But this was still not satisfactory and a new kind of breathing system was evolved. A powerful set of muscles, known as the diaphragm, allowed a much more forceful system for inspiring and then exhaling air.
Thus, by the middle Triassic, some very different respiratory designs were in play, with natural selection and competition as the arbiters. We know what kind of lungs the mammals had. But what about the diapsids—and their most famous members (and descendents of the early kinds such as Proterosuchus)—the dinosaurs? By the end of the middle Triassic they had burst upon the scene. How did they breathe? Therein lies a controversy.
What kind of lungs did the earliest dinosaurs evolve? What changes to this lung design came about in their descendents? This has been the source of controversy for more than two decades now. But before we enter the debate, it should be noted that whichever lung system was found in the Triassic dinosaurs, it evolved for a self-similar